Lateral heterojunction bipolar transistor and method of fabricating the same

ABSTRACT

A lateral heterojunction bipolar transistor comprises a first semiconductor layer in a mesa configuration disposed on an insulating layer, a second semiconductor layer formed by epitaxial growth on the side surfaces of the first semiconductor layer and having a band gap different from that of the first semiconductor layer, and a third semiconductor layer formed by epitaxial growth on the side surfaces of the second semiconductor layer and having a band gap different from that of the second semiconductor layer. The first semiconductor layer serves as a collector of a first conductivity type. At least a part of the second semiconductor layer serves as an internal base layer of a second conductivity type. At least a part of the third semiconductor layer serves as an emitter operating region of the first conductivity type. The diffusion of an impurity is suppressed in the internal base formed by epitaxial growth.

BACKGROUND OF THE INVENTION

The present invention relates to a lateral heterojunction bipolartransistor and to a method of fabricating the same. More particularly,it relates to a lateral heterojunction bipolar transistor having aheterostructure such as Si/Si_(1-x)Ge_(x) or Si/Si_(1-x-y)Ge_(x)C_(y)formed on an insulating substrate such as SOI (Silicon on Insulator).

There has conventionally been proposed technology for providing atransistor with excellent characteristics by forming a CMOS device and abipolar transistor on a SOI (Silicon on Insulator) substrate composed ofa silicon layer stacked on an insulating layer to lower the operatingvoltage of the transistor, provide a complete isolation between devices,and reduce a parasitic capacitance. In transmitting/receiving portionsof a communication device handling an RF signal, in particular, acrosstalk between an analog circuit and a digital circuit presents aproblem. However, the use of the SOI substrate holds promise of removingthe crosstalk more drastically than the conventional technology.

On the other hand, a heterojunction bipolar transistor using aheterostructure such as Si/SiGe has been used commercially in recentyears as a device operable in a region of RF frequencies, which has beenconsidered difficult to fabricate by using the technology using asilicon process. Compared with a Si homojunction bipolar transistor, theheterojunction bipolar transistor has an excellent characteristic suchthat the resistance of a base can be reduced by adjusting the impurityconcentration in the base to be higher than in the Si homojunctionbipolar transistor since reverse injection of carriers from the base toan emitter is suppressed by using the heterostructure in which the bandgap of the base is smaller than the band gap of the emitter.

In response to the system-on-chip demand made in recent days, BiCMOStechnology has been requested to form a CMOS device and a bipolartransistor on a single chip. To form the bipolar transistor on a SOIsubstrate, however, it is necessary to increase the thickness of asilicon layer to a certain degree in a conventional vertical bipolartransistor structure, while it is necessary to reduce the thickness ofthe silicon layer in the CMOS device for high-speed operation and thesuppression of a leakage current. However, the provision of a siliconlayer having different thicknesses in a CMOS device region and a bipolartransistor region increases the complexity of the fabrication process.

To use a silicon layer having the same thickness in the bipolartransistor region as in the CMOS device region, there has been proposedthe formation of a lateral heterojunction bipolar transistor on a SOIsubstrate. By using a lateral heterojunction bipolar transistorstructure, the silicon layer having the same thickness in the bothregions can be used and the process steps are greatly reduced in number.It has also been reported that a parasitic resistance is smaller in thelateral heterojunction bipolar transistor structure than in the verticalbipolar transistor formed by using a SOI substrate, which isadvantageous in terms of high-speed operation.

FIGS. 10(a) and 10(b) are a plan view and a cross-sectional view of alateral heterojunction bipolar transistor provided on a SOI disclosed ina document about an example of a prototype of such a lateralheterojunction bipolar transistor (A 31 GHz f_(max) Lateral BJT on SOIUsing Self-Aligned External Base Formation Technology: T. Shino et. al.1998 IEEE). As shown in the drawings, the lateral heterojunction bipolartransistor is formed on a SOI substrate including a BOX layer 1001composed of a silicon oxide film and a silicon layer 1009. By using theSOI substrate, a parasitic capacitance in the operating region of thetransistor can be reduced. The thickness of the silicon layer 1009 is0.1 μm. The silicon layer 1009 comprises: a strip-like p-type internalbase layer 1004 doped with boron (B); two external base layers 1006connected to the shorter side portions on both ends of the internal baselayer 1004 and doped with boron (B) at a concentration higher than thatin the internal base layer 1004; and an n-type emitter 1005 and ann-type collector 1002 disposed with the longer side portions of theinternal base layer 1004 interposed therebetween. The emitter 1005 hasbeen doped with arsenic (As) at a high concentration and the collector1002 has been doped with arsenic at a non-uniform concentration. Inshort, the collector 1002 has a retrograde structure in which theconcentration of arsenic is lower for an increased breakdown voltage inthe portions thereof closer to the internal base layer 1004 and theexternal base layers 1006, which increases gradually with distance fromthe internal base layer 1004 and the external base layers 1006. Therespective electrode formation portions of the external base layers1006, the emitter 1005, and the collector 1002 are located on therespective outward tips of the regions such that the longest possibledistances are provided therebetween and that parasitic capacitancesamong base electrodes, an emitter electrode, and a collector electrodeare reduced. The foregoing document reports that such a lateralheterojunction bipolar transistor has provided a maximum oscillationfrequency fmax of 31 GHz.

FIGS. 11(a) to 11(e) are perspective views illustrating a method offabricating the bipolar transistor disclosed in the document.

First, in the step shown in FIG. 11(a), an oxide film and a SiN film(not shown) are formed on the n-type silicon layer 1009 into whichphosphorus (P) has been introduced. Then, an array-like resist mask 1108is formed on the SiN to cover an NPN active region. Subsequently, boron(B) is ion implanted at a dose of 4×10¹⁵ atoms·cm⁻² into the siliconlayer 1009 except for the NPN active region 1107 from above the resistmask 1108, whereby a P⁺ diffused region is formed. Next, in the stepshown in FIG. 11(b), the SiN film is patterned by using the resist mask1108 as a mask and side etched to form a SiN mask 1110, which isinwardly offset by about 0.2 μm from the ends of the resist mask 1108.Thereafter, the resist mask 1108 is removed. Then, in the step shown inFIG. 11(c), a TEOS mask 1111 is formed in crossing relation to the SiNmask 1110. Subsequently, boron (B) is ion implanted at a dose of 1×10¹⁴atoms·cm⁻² and an acceleration energy of 25 keV into the silicon layer1009 except for the region covered with the SiN mask 1110 and the TEOSmask 1111. Next, in the step shown in FIG. 11(d), the SiN mask 1110 andthe TEOS mask 1111 are removed. At this time, the width of the internalbase layer 1004 is determined by the diffusion distance traveled byimplanted boron, which is measured from the end portion of the TEOS mask1111. Finally, in the step shown in FIG. 11(e), portions serving as theemitter and the collector are mesa etched and arsenic (As) is ionimplanted into the respective portions at a dose of 1×10¹⁵ atoms·cm⁻²and an acceleration voltage of 120 keV and at a dose of 1×10¹⁵atoms·cm⁻² and an acceleration voltage of 65 keV. Since the siliconlayer 1009 is amorphized by the ion implantations, it is recrystallizedby RTA performed at 1050° C. for 20 sec and by electric furnaceannealing performed at 850° C. for 60 sec.

By the foregoing process, a lateral bipolar transistor with a smallparasitic capacitance which is high in fmax and operable at a high speedcan be formed.

However, since the width of the internal base 1104 is determined by thediffusion distance of boron in accordance with the prior art technologydisclosed in the foregoing document, it is difficult to constantlyobtain a desired impurity distribution. Since the range in which theemitter 1105 and the collector 1102 are formed is determined by thediffusion distance of the n-type impurity, it is also difficult to forma pn junction with a sharp impurity concentration distribution.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide, duringthe formation of a lateral heterojunction bipolar transistor on a SOIsubstrate, means for accurately adjusting the width of an internal baselayer or the like to a desired dimension and thereby provide a lateralheterojunction bipolar transistor having stable characteristics and afabrication method therefor.

A first lateral heterojunction bipolar transistor comprises: a substratehaving an insulating layer; a first semiconductor layer in a mesaconfiguration disposed on the insulating layer; a second semiconductorlayer formed by epitaxial growth on a side surface of the firstsemiconductor layer, the second semiconductor layer having a band gapdifferent from a band gap of the first semiconductor layer; and a thirdsemiconductor layer formed by epitaxial growth on a side surface of thesecond semiconductor layer, the third semiconductor layer having a bandgap different from the band gap of the second semiconductor layer, atleast a part of the second semiconductor layer functioning as aninternal base layer of a second conductivity type.

In the arrangement, the lateral thickness of the second semiconductorlayer serving as the internal base layer is determined by epitaxialgrowth, not by the implantation of impurity ions. Consequently, thelateral thickness of the internal base layer is controlled with highaccuracy. Since the internal base layer is formed by epitaxial growth,not by the implantation of impurity ions, the internal base layer can bedoped in situ with an impurity, while it is laterally grown. Thisprovides a sharp impurity concentration distribution in which impuritydiffusion is suppressed.

There can be adopted a structure in which least the first semiconductorlayer functions as a collector of a first conductivity type and at leasta part of the third semiconductor layer functions as an emitteroperating region of the first conductivity type.

The first lateral heterojunction bipolar transistor further comprises anexternal base layer of the second conductivity type in contact with thesecond semiconductor layer. The arrangement allows easy formation of anelectrode.

The band gap of the second semiconductor layer is smaller than the bandgap of the third semiconductor layer. The arrangement suppresses reverseinjection of carriers from the second semiconductor layer functioning asthe internal base layer into the third semiconductor layer functioningas the emitter operating region. As a result, base resistance can bereduced by adjusting an impurity concentration in the secondsemiconductor layer higher than a concentration in a homojunctionbipolar transistor.

Each of the first and third semiconductor layers is composed of asilicon layer and the second semiconductor layer is composed of an alloycontaining at least any two of Si, Ge, and C. The arrangement allows theformation of a heterojunction bipolar transistor in which impuritydiffusion is suppressed by using a silicon process.

A principal surface of the first semiconductor layer is a {110} planeand a side surface of the first semiconductor layer in contact with thesecond semiconductor layer is a {111} plane. The arrangement providesthe first semiconductor layer with a smooth side surface by using wetetching.

A first method of fabricating a lateral heterojunction bipolartransistor comprises the steps of: (a) forming an etching mask on asemiconductor layer disposed on an insulating layer to compose asubstrate; (b) patterning the semiconductor layer by etching includingdry etching and using the etching mask to form a first semiconductorlayer in a mesa configuration; (c) epitaxially growing, on at least oneside surface of the first semiconductor layer, a second semiconductorlayer having a band gap different from a band gap of the firstsemiconductor layer; and (d) epitaxially growing, on a side surface ofthe second semiconductor layer, a third semiconductor layer having aband gap different from the band gap of the second semiconductor layer,at least the first semiconductor layer functioning as a collector of afirst conductivity type, at least a part of the second semiconductorlayer functioning as an internal base layer of a second conductivitytype, at least a part of the third semiconductor layer functioning as anemitter operating region of the first conductivity type.

In accordance with the method, the lateral thickness of the firstsemiconductor layer functioning as the internal base layer is determinedby epitaxial growth, not by the implantation of impurity ions.Consequently, the lateral thickness of the internal base layer iscontrolled with high accuracy. Since the internal base layer is formedby epitaxial growth, not by the implantation of impurity ions, theinternal base layer can be doped in situ with an impurity, while it islaterally grown. This provides a sharp impurity concentration diffusionin which impurity diffusion is suppressed.

The step (b) includes: patterning the semiconductor layer by dry etchinginto a configuration of the etching mask and; forming the firstsemiconductor layer by performing wet etching with respect to a sideportion of the patterned semiconductor layer, while leaving the etchingmask. The arrangement is preferred since it removes etching damage,while retaining a high patterning accuracy.

The first method further comprises, after the step (d), the steps of:(e) depositing a polycrystalline semiconductor film on the substrate;and (f) planarizing the polycrystalline semiconductor film by CMP toform an emitter in contact with at least the third semiconductor layer.This allows easy formation of a low-resistance emitter adjacent theemitter operating region.

The first method further comprises, in or after the step (e), the stepof: (g) introducing an impurity of the first conductivity type into afirst region of the polycrystalline semiconductor film and introducingan impurity of the second conductivity type into a second region of thepolycrystalline semiconductor film; and removing, of the polycrystallinesemiconductor film, at least a portion located between the first andsecond regions to form an emitter in contact with the thirdsemiconductor layer from the first region and form an external baselayer in contact with the second semiconductor layer from the secondregion. This allows easy formation of a low-resistance emitter and alow-resistance external base layer by using a polycrystalline film suchas a polysilicon film.

Preferably, the introduction of the impurity is performed by ionimplantation using a mask.

Preferably, the step (g) is performed by wet etching.

The etching mask is formed by using a semiconductor layer having aprincipal surface of a {110} plane as the semiconductor layer on theinsulating layer in the step (a) and such that the side surface of thefirst semiconductor layer in contact with the second semiconductor layeris a {111} plane in the step (b). This allows the formation of aninternal base layer having a uniform lateral thickness by using a {111}plane which is etched at a particularly low speed and provides a smoothflat surface.

Preferably, the step (b) includes: crystal anisotropic etching using anetching solution containing at least any one of ethylenediamine,pyrocatechol, KOH, and hydrazine.

A second lateral heterojunction bipolar transistor disposed on aninsulating layer comprises: a first semiconductor layer functioning as acollector; a second semiconductor layer disposed in contact with atleast one side surface of the first semiconductor layer to function asan internal base having a band gap smaller than a band gap of the firstsemiconductor layer; a third semiconductor layer disposed in contactwith a side surface of the second semiconductor layer to function as anemitter having a band gap larger than the band gap of the secondsemiconductor layer; first and second electrodes in contact withrespective side surfaces of the first and third semiconductor layers;and a third electrode disposed in contact with a top surface of thesecond semiconductor layer.

This provides a lateral heterojunction bipolar transistor having arelatively simple structure and excellent characteristics of lowparasitic capacitance, low parasitic resistance, and low baseresistance, which is formed on the insulating layer.

Each of the first and second electrodes is composed of a metal. Thisparticularly lowers the resistances of the emitter and collector.

A second method of fabricating a lateral heterojunction bipolartransistor comprises the steps of: (a) introducing an impurity of afirst conductivity type into a first semiconductor layer containing animpurity of the first conductivity type, the first semiconductor layerbeing disposed on an insulating layer to compose a substrate; (b)forming, on the first semiconductor layer, an etching mask having a slitwith a width of 200 nm or less; (c) removing a portion of thesemiconductor layer located under the slit by etching using the etchingmask to form a groove penetrating the first semiconductor layer; (d)epitaxially growing, from both side surfaces of the groove in the firstsemiconductor layer, a second semiconductor layer having a band gapdifferent from a band gap of the first semiconductor layer such that thesecond semiconductor layer is buried in the groove; (e) forming openingsin respective regions of the insulating layer located on both sides ofthe slit and above the first semiconductor layer; (f) performing wetetching with respect to the first semiconductor layer from the openingsin the insulating layer to form hollow portions and leave respectiveportions of the first semiconductor layer on both sides of the secondsemiconductor layer; (g) forming first and second electrodes to beburied in the respective hollow portions; and (h) forming a thirdelectrode to be buried in the slit in the insulating film in contactrelation with the second semiconductor layer, the respective portions ofthe first semiconductor layer left on both sides of the secondsemiconductor layer functioning as a collector and an emitter operatingregion, the second semiconductor layer functioning as an internal baselayer.

The method provides a lateral heterojunction bipolar transistor having arelatively simple structure and excellent characteristics of lowparasitic capacitance, low parasitic resistance, and low baseresistance, which is formed on the insulating layer.

Preferably, the step (f) includes: crystal anisotropic etching using atleast any one of ethylenediamine, pyrocatechol, KOH, and hydrazine.

The step (a) includes a first ion implantation for implanting impurityions of the first conductivity into the first semiconductor layer and asecond ion implantation for implanting, into a portion of the firstsemiconductor layer, the impurity ions at a concentration higher than inthe first ion implantation, the collector is formed from a portion ofthe first semiconductor layer with respect to which only the first ionimplantation has been performed and the second ion implantation has notbeen performed, and the emitter operating region is formed from theportion of the first semiconductor layer with respect to which the firstand second ion implantations have been performed. This allows respectiveimpurity concentrations in the emitter operating region and in thecollector to be adjusted optimally for the operation of the bipolartransistor.

A silicon layer is used as the first semiconductor layer and an alloycontaining at least any two of Si, Ge, and C is used as the secondsemiconductor layer. This allows the fabrication of a lateralheterojunction bipolar transistor using a silicon process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) and 1(b) are a plan view and a perspective view of a lateralheterojunction bipolar transistor according to a first embodiment of thepresent invention;

FIGS. 2(a) to 2(h) are cross-sectional views illustrating a method offabricating the lateral heterojunction bipolar transistor according tothe first embodiment;

FIGS. 3(a) and 3(b) illustrate a lateral impurity profile in the regionA shown in FIG. 2(h) of the lateral heterojunction bipolar transistoraccording to the first embodiment;

FIGS. 4(a) and 4(b) illustrate a lateral impurity profile in the regionB shown in FIG. 2(h) of the lateral heterojunction bipolar transistoraccording to the first embodiment;

FIG. 5 is a plan view of a lateral heterojunction bipolar transistoraccording to a second embodiment of the present invention;

FIGS. 6(a) and 6(b) are a plan view and a cross-sectional view of alateral heterojunction bipolar transistor according to a thirdembodiment of the present invention;

FIGS. 7(a) to 7(e) are cross-sectional views illustrating a method offabricating the lateral heterojunction bipolar transistor according tothe third embodiment;

FIG. 8 is a plan view of a lateral heterojunction bipolar transistoraccording to a fourth embodiment of the present invention;

FIGS. 9(a) to 9(f) are cross-sectional views illustrating a method offabricating the lateral heterojunction bipolar transistor according tothe fourth embodiment;

FIGS. 10(a) and 10(b) are a plan view and a cross-sectional view of aconventional lateral heterojunction bipolar transistor disclosed in thedocument; and

FIGS. 11(a) to 11(e) are cross-sectional views illustrating a method offabricating the conventional lateral heterojunction bipolar transistor.

DETAILED DESCRIPTION OF THE INVENTION Embodiment 1

FIGS. 1(a) and 1(b) are a plan view and a perspective view of a lateralheterojunction bipolar transistor according to a first embodiment of thepresent invention.

As shown in FIGS. 1(a) and 1(b), the lateral heterojunction bipolartransistor according to the present embodiment has a so-called SOIstructure comprising: a Si substrate 150; a BOX layer 151 composed of asilicon oxide film disposed on the Si substrate 150; and a semiconductorlayer 152 disposed on the BOX layer 151. The semiconductor layer 152comprises: a collector 101 having a generally square plan configurationand made of n-type single-crystal silicon; a SiGeC/Si layer 102 composedof a p-type SiGeC layer and an n-type Si layer each having an annularconfiguration surrounding the collector layer 101; an emitter 103 madeof n-type polysilicon; and a p-type polysilicon layer 105. The portionof the SiGeC/Si layer 102 interposed between the collector 101 and theemitter 103 and composed of the p-type SiGeC layer (the portion internalof the broken line in the drawing) forms an internal base layer 102 aand the portion of the SiGeC/Si layer 102 interposed between thecollector 101 and the emitter 103 and composed of the n-type Si layer(the portion external of the broken line in the drawing forms an emitteroperating region 102 b. The portion 102 c of the SiGeC/Si layer 102except for the internal base layer 102 a and the emitter operatingregion 102 b and the p-type polysilicon layer 105 constitute an externalbase 104.

The collector 101 with a thickness of about 200 nm and sides of about0.6 μm has been doped with antimony (Sb) (which may be phosphorous orarsenic) at a concentration of about 1×10¹⁹ atoms·cm⁻³. The principalsurface of the collector 101 is a (110) plane and each of the sidesurfaces thereof is a smooth (111) plane. It is to be noted thatprincipal surface of the collector 101 need not be a (110) plane andeach of the side surfaces thereof need not be a (111) plane. Althoughthe internal base 102 a contains boron at a concentration of about2×10¹⁸ atoms·cm⁻³ and is composed of a Si_(1-x)Ge_(x)C_(y) layer havinga graded composition in the present embodiment, the internal base 102 amay also be composed of SiGe containing no C (such as Si_(1-x)Ge_(x) orthe like having a graded composition). However, the presence of Ccontained only in an extremely small amount achieves the particularlylarge effect of preventing the diffusion of the impurity. On the otherhand, the emitter operating region 102 b is made of single-crystal Sicontaining phosphorus at a concentration of about 1×10¹⁸ atoms·cm⁻³. Theemitter 103 is made of n-type polysilicon containing phosphorus at aconcentration of about 1×10²⁰ atoms·cm⁻³ or more. It is to be noted thatarsenic may also be used instead of phosphorus for doping. That is, aSi/SiGeC/Si heterojunction is formed among the emitter operating region,the internal base, and the collector. The external base 104 is composedof polysilicon containing boron at a concentration of about 1×10²⁰atoms·cm⁻³. The external base 104 functions as a contact region with theinternal base 102 a.

The collector 101 has a retrograde structure in which the concentrationof the n-type impurity (antimony) increases gradually with distance fromthe internal base 102 a. The internal base 102 a has a gradedcomposition such that the content of Ge (or Ge and C) decreasesgradually with distance from the collector 101. However, the retrogradein the collector 101 and the graded composition in the internal base 102a need not necessarily be provided.

Referring to FIGS. 2(a) to 2(h), a method of fabricating the lateralheterojunction bipolar transistor according to the present embodimentwill be described.

First, in the step shown in FIG. 2(a), the SOI substrate composed of theSi substrate 150, the BOX layer 151 composed of the silicon oxide film,and the Si film formed on the BOX layer 151 is formed. To form the SOIsubstrate, any well-known method (such as the SIMOX method) may beadopted. The present embodiment has adopted a method of bonding asilicon wafer to a silicon oxide film formed on a surface thereof and asilicon wafer such that the silicon oxide film is sandwiched between theboth silicon wafers and thinning one of the silicon wafers by polishing.The Si film on the BOX layer 151 is preliminarily doped with antimony(which may be arsenic or phosphorus) at a concentration of about 1×10¹⁹atoms·cm⁻³. Then, the Si film is patterned to form the square collector101 (mesa portion) with corners rounded off. At this time, each of theside surfaces of the collector 101 can be formed to an extremely smooth(111) plane by performing wet etching using a square resist mask formedon the Si film having a principal surface of a (110) plane and havingsides parallel to the <211> direction and thereby using the anisotropicproperty of an etch speed resulting from crystal orientation.Alternatively, the collector 101 may also be formed by forming, on theSi film, an etching mask covering the collector layer 101 and performingdry etching using the etching mask.

Next, in the step shown in FIG. 2(b), an undoped Si layer having athickness of about 120 nm and partly composing the collector 101 isepitaxially grown on the side surfaces of the mesa portion of thecollector 101 by CVD (Chemical Vapor Deposition) or by UHV-CVD (UltraHigh Vacuum-CVD). During the step, antimony (Sb) is diffused from themesa portion of the collector 101 into the Si layer in the epitaxialstep, thereby forming a retrograde impurity concentration profile.Thereafter, a SiGeC layer having a lateral thickness of about 80 nm isepitaxially grown, while it is doped in situ with boron at aconcentration of about 2×10¹⁸ atoms·cm⁻³ such that the content of C isconstant (about 2%) and the content of Ge is graded as shown in FIG. 3,which will be described later. Then, an undoped Si layer having alateral thickness of about 10 nm is formed, whereby the SiGeC/Si layer102 is formed. Since the SiGeC layer of the SiGeC/Si layer 102 contains2% of C, the diffusion of boron in the subsequent heat treatment stepcan be prevented more positively and a heterojunction portion having asharper impurity concentration profile can be implemented.

Next, in the step shown in FIG. 2(c), a polysilicon film 160 isdeposited on the substrate. Then, in the step shown in FIG. 2(d), thepolysilicon film 160 is etched back and thereby planarized by a methodsuch as CMP (Chemical Mechanical Polishing).

Next, in the step shown in FIG. 2(e), an oxide film 161 is formed on thesubstrate. Subsequently, boron ions are implanted into the portion ofthe polysilicon film 160 serving as the external base 104 such that theconcentration of boron is about 1×10²⁰ atoms·cm⁻³ or more, while theportion of the oxide film 161 serving as the emitter 103 is doped withphosphorus (which may be arsenic or antimony) ions such that theconcentration of phosphorus is about 1×10²⁰ atoms·cm⁻³ or more. It is tobe noted that the Si substrate 150 and the BOX layer 151 are notdepicted in the drawings showing the steps subsequent to the step shownin FIG. 2(e).

Next, since a leakage current may flow between the external base 104 andthe emitter 103 via the undoped portion of the polysilicon film 160 inthe current configuration, the polysilicon layer 160 is partiallyremoved by the following process such that the external base 104 and theemitter 103 are electrically insulated from each other. Specifically, inthe step shown in FIG. 2(f), an opening is formed in the portion of theoxide film 161 at a specified distance from the portion of thepolysilicon film 160 into which the impurity ions have been implanted.Then, in the step shown in FIG. 2(g), the polysilicon film 160 is etchedby wet etching till the SiGeC/Si layer 102 is reached. In this case, anetching solution having a high etching selectivity between polysiliconand Si is used, whereby insulation is provided without damaging theSiGeC/Si layer 102. Thereafter, a heat treatment (annealing) foractivating the implanted impurity ions is performed. By the heattreatment, the n-type impurity such as phosphorus implanted in thepolysilicon of the emitter 103 is diffused into the undoped silicon ofthe emitter operating region 102 b, so that the emitter operating region102 b functions as the emitter region of the npn bipolar transistor.

Next, in the step shown in FIG. 2(h), the oxide film 161 is removed,whereby the lateral heterojunction bipolar transistor having thestructure shown in FIG. 1(b) is obtained.

FIGS. 3(a) and 3(b) illustrate a lateral impurity profile in the regionA shown in FIG. 2(h) of the lateral heterojunction bipolar transistoraccording to the present embodiment. FIGS. 4(a) and 4(b) illustrate alateral impurity profile in the region B shown in FIG. 2(h) of thelateral heterobipolar transistor according to the present embodiment.

As shown in FIGS. 3(a) and 3(b) and FIGS. 4(a) and 4(b), a retrogradedistribution has been formed in the collector 101, in which theconcentration of Sb as the impurity increases gradually with distancefrom the internal base 102 a to provide a higher breakdown voltage. Thecontent of Ge in the internal base 102 a is graded such that a driftelectric field is generated. Although phosphorus in the emitter 103composed of polysilicon is constantly at a high concentration of about5×10²⁰ atoms·cm⁻³, it has been diffused into the internal base 102 a.The concentration after the diffusion is preferably minimized. Althoughthe external base 104 has been doped with boron at a high concentration,the external base 104 is electrically integrated with the internal base102 a and held at substantially the same potential as the internal base102 a, since the external base 104 has been doped with the impurity ofthe same polarity as the internal base 102 a.

Since the present embodiment has determined the lateral thickness of theinternal base layer 102 a by epitaxial growth by in-situ doping, not bythe implantation of impurity ions, the lateral thickness of the internalbase layer 102 a is not dependent on the accuracy of photolithography oron the degree of impurity diffusion. Since the internal base layer 102 ais formed by epitaxial growth by in-situ doping, not by the implantationof impurity ions, the diffusion of the impurity is suppressed and arelatively sharp impurity concentration distribution is obtained. Inaddition, since the internal base layer 102 a is composed of the SiGeClayer in the present embodiment, the diffusion of the impurity in theheat treatment step is suppressed by the presence of C and the impurityconcentration profile is maintained without being deformed. Even if theinternal base layer 102 a is composed of a SiGe layer instead of theSiGeC layer, the effect of properly maintaining the impurityconcentration profile is achieved to a certain degree, since the speedat which the impurity is diffused in the SiGe layer is lower than thespeed at which the impurity is diffused in the Si layer.

Moreover, since the lateral heterojunction bipolar transistor of thepresent embodiment uses the SiGeC/Si heterojunction, it can achieve thefollowing effect by contrast to the lateral heterojunction bipolartransistor using the Si homojunction disclosed in the foregoingdocument. That is, since the band gap of the internal base layer issmaller than the band gap of the emitter operating region, reverseinjection of carriers from the internal base layer into the emitteroperating region is suppressed. As a result, it becomes possible toreduce base resistance by adjusting the impurity concentration in theinternal base layer higher than the concentration in the homojunctionbipolar transistor.

Since the present embodiment uses the SOI substrate, it can provide alateral heterojunction bipolar transistor with a small parasiticcapacitance which is high in fmax and suitable for high-speed operation,similarly to the technology disclosed in the foregoing document.

In the structure shown in FIG. 1, the mesa single-crystal Si layerdesignated at 101 may also serve as the emitter, not as the collector,the polysilicon layer designated at 103 may also serve as a collectorwithdrawn layer, not as the emitter, and the single-crystal Si layerdesignated at 102 b may also serve as the collector. In this case, abipolar transistor with a particularly high breakdown voltage isobtained. In addition, the single-crystal Si region serving as thecollector preferably has a lateral thickness of 0.2 μm or more and aretrograde distribution is formed more preferably, in which theconcentration of Sb as the impurity increases gradually with distancefrom the internal base 102 a to provide a higher breakdown voltage,similarly to the collector of the present embodiment.

Embodiment 2

A description will be given below to a second embodiment as a variationof the lateral heterojunction bipolar heterojunction transistoraccording to the first embodiment.

FIG. 5 is a plan view of a lateral heterojunction bipolar transistoraccording to the second embodiment. In the present embodiment, thestructure of a portion functioning as an npn transistor is the same asin the first embodiment.

As shown in the drawings, the lateral heterojunction bipolar transistoraccording to the present embodiment also has a so-called SOI structurecomprising: a Si substrate; a BOX layer composed of a silicon oxide filmdisposed on the Si substrate; and a semiconductor layer disposed on theBOX layer. In the semiconductor layer, a linear SiGeC/Si layer 112composed of a p-type SiGeC layer and an n-type Si layer is provided. Acollector 111 made of single-crystal silicon containing an n-typeimpurity and an emitter 113 made of polysilicon containing an n-typeimpurity are disposed on both sides of the SiGeC/Si layer 112. Externalbases 114 each composed of a polysilicon layer containing a p-typeimpurity is disposed on both ends of the middle straight line portion ofthe SiGeC/Si layer 112. Of the SiGeC/Si layer 112, the portion composedof the p-type SiGeC layer (hatched portion in the drawing) is aninternal base layer 112 a and the portion composed of the n-type Silayer (the blank portion in the drawing) is the emitter operating region112 b.

The collector layer 111 with a thickness of about 200 nm and sides ofabout 1.0 μm has been doped with antimony (which may be phosphorous orarsenic) at a concentration of about 1×10¹⁹ atoms·cm⁻³. The principalsurface of the collector 111 is a (110) plane and each of the sidesurfaces thereof is a smooth (111) plane. Although the internal base 112a contains boron at a concentration of about 2×10¹⁸ atoms·cm⁻³ and iscomposed of a Si_(1-x)Ge_(x)C_(y) layer having a graded composition inthe present embodiment, the internal base 112 a may also be composed ofSiGe containing no C (such as Si_(1-x)Ge_(x) having a gradedcomposition). However, the presence of C contained in an extremely smallamount achieves the particularly large effect of preventing thediffusion of the impurity. On the other hand, the emitter operatingregion 112 b is made of single-crystal Si containing phosphorus (orarsenic) at a concentration of about 1×10¹⁸ atoms·cm⁻³ or more. Theemitter 113 is composed of n-type polysilicon containing phosphorus (orarsenic) at a concentration of about 1×10²⁰ atoms·cm⁻³ or more. That is,a Si/SiGeC/Si heterojunction is formed among the emitter operatingregion, the internal base, and the collector. The external base 114 iscomposed of polysilicon containing boron at a concentration of about1×10²⁰ atoms·cm⁻³. The external base 114 functions as a contact regionwith the internal base 112 a. The external base 114 and the collector111 are electrically insulated from each other by a first insulatingfilm 115. The external base 114 and the emitter 113 are electricallyinsulated from each other by a second insulating film 116.

The collector 111 has a retrograde structure in which the concentrationof the n-type impurity (antimony) increases gradually with distance fromthe internal base 112 a. The internal base 112 a has a gradedcomposition such that the content of Ge (or Ge and C) decreasesgradually with distance from the collector 111, which increases themobility of electrons in the internal base layer 112 a. However, theretrograde structure in the collector 111 and the graded composition inthe internal base 112 a need not necessarily be provided.

A method of fabricating the lateral heterojunction bipolar transistoraccording to the present embodiment will be described briefly.

Although the lateral heterojunction bipolar transistor according to thepresent embodiment has a plan configuration different from that of thelateral heterojunction bipolar transistor according to the firstembodiment, the basic fabrication process is the same as in the firstembodiment. That is, the SOI substrate composed of the Si substrate, theBOX layer, and the Si film is formed and then the Si film is patternedto form the mesa portion of the collector 111. At this time, each of theside surfaces of the middle portion of the collector 111 can be formedto an extremely smooth (111) plane by the same process as performed inthe first embodiment. Then, one of the side surfaces of the mesa portionof the collector 111 is exposed by covering the other side surfacesthereof with the first insulating film 115. Subsequently, an undoped Silayer partly composing the collector 111 is grown epitaxially on the oneside surface by CVD or UHV-CVD. Then, the SiGeC layer containing 2% of Cin which the content of Ge is graded is epitaxially grown on the undopedSi layer. By further forming an undoped Si layer thereafter, theSiGeC/Si layer 112 is formed. After that, a polysilicon film isdeposited on the substrate, etched back, and thereby planarized. Afterboron ions are implanted into the portion of the polysilicon filmserving as the external base 114 and phosphorus ions are implanted intothe portion thereof serving as the emitter 113, the patterning of thepolysilicon film and the burying of an insulator is performed, wherebythe emitter 113 and the external base 114 are electrically insulatedfrom each other by the second insulating film 116.

Thereafter, a heat treatment (annealing) for activating the implantedimpurity ions is performed. By the heat treatment, an n-type impuritysuch as phosphorus implanted in the polysilicon of the emitter 113 isdiffused into the undoped silicon of the emitter operating region 112 bsuch that the emitter operating region 112 b functions as the emitterregion of the npn bipolar transistor. In the Si layer of the epitaxiallayers partly composing the collector 111, antimony (Sb) is diffusedfrom the mesa portion of the collector 111 to form a retrograde impurityconcentration profile.

In the present embodiment also, the conditions for impurity implantationand the types of ions implanted in the foregoing fabrication process arethe same as in the first embodiment.

Although the present embodiment achieves basically the same effects asachieved by the first embodiment, the first embodiment is advantageousover the second embodiment in that an area occupied by the entirebipolar transistor is smaller.

Embodiment 3

FIGS. 6(a) and 6(b) are a plan view and a cross-sectional view of alateral heterojunction bipolar transistor according to a thirdembodiment of the present invention.

As shown in FIGS. 6(a) and 6(b), the lateral heterojunction bipolartransistor according to the present embodiment has a so-called SOIstructure comprising: a Si substrate 250; a BOX layer 251 composed of asilicon oxide film disposed on the Si substrate 250; and a semiconductorlayer 252 disposed on the BOX layer 251. In the semiconductor layer 252,an internal base layer 202 a composed of a p-type SiGe layer having alinear plan configuration is provided. A collector 201 a made of n-typesingle-crystal silicon and an emitter 203 a made of n-typesingle-crystal silicon are disposed on both sides of the internal baselayer 202 a. The lateral heterojunction bipolar transistor according tothe present embodiment also comprises: an oxide film 206 covering thetop surface of the semiconductor layer 252; an external base layer 202 bmade of p-type polysilicon in contact with the internal base layer 202 athrough an opening in the oxide film 206; a collector contact 201 b madeof n-type polysilicon buried in a groove formed in the oxide film 206and the collector 201 a; and an emitter contact 203 b made of n-typepolysilicon buried in a groove formed in the oxide film 206 and theemitter 203 a.

Although the principal surface of each of the collector 201 and theemitter 203 is a (100) plane in the present embodiment, the principalsurface of each of the collector 201 and the emitter 203 may also be a(110) plane and each of the side surfaces thereof may also be a smooth(111) plane, similarly to the first and second embodiments. Thecollector 201 and the emitter 203 have been doped with phosphorus at aconcentration of about 1×10¹⁸ atoms·cm⁻³. Although the internal baselayer 202 a contains boron at a concentration of about 5×10¹⁸ atoms·cm⁻³and is composed of the SiGe layer having a composition represented bySi_(0.7)Ge_(0.3) in the present embodiment, an extremely small amount(e.g., about 2%) of C may also be contained in the internal base layer202 a. However, the presence of C contained in only an extremely smallamount achieves the particularly large effect of preventing thediffusion of the impurity. Each of the collector contact 201 b, theemitter contact 203 b, and the external base layer 202 b has been dopedwith phosphorus at a concentration of about 1×10²⁰ atoms·cm⁻³ or more.

Referring to FIGS. 7(a) to 7(e), a method of fabricating the lateralheterojunction bipolar transistor according to the present embodimentwill be described. FIGS. 7(a) to 7(e) are cross-sectional viewsillustrating the process of fabricating the lateral heterojunctionbipolar transistor according to the present embodiment.

First, in the step shown in FIG. 7(a), the SOI substrate composed of theSi substrate 250, the BOX layer 251 composed of the silicon oxide film,and the Si film (semiconductor layer) formed on the BOX layer 251 isformed. The semiconductor layer 252 has a thickness of about 200 nm andhas been doped with phosphorus at a concentration of about 1×10¹⁸atoms·cm⁻³.

Next, in the step shown in FIG. 7(b), the oxide film 206 is formed onthe semiconductor layer 252 and a slit 207 is formed in the middleportion of the oxide film 206. The slit 207 is then increased in depthto penetrate the semiconductor layer 252 in the step shown in FIG. 7(c).

Next, in the step shown in FIG. 7(d), Si_(0.7)Ge_(0.3) is epitaxiallygrown by CVD or UHV-CVD from both sides of the slit 207 to be united inthe middle of the slit 207, thereby forming the internal base layer 202a buried in the slit 207. During the step, the internal base layer 202 ais doped in situ with boron at a concentration of about 5×10¹⁸atoms·cm⁻³. Thereafter, grooves are formed by dry etching in the regionsof the oxide film 206 located on both sides of the slit and at adistance of about 200 nm therefrom. The grooves are further enlarged bywet etching to reach the semiconductor layer 252 to form grooves 208 and209 therein. During the step, the isotropic etching action of wetetching laterally enlarges the grooves 208 and 209 till the end portionof each of the grooves 208 and 209 reaches a point at a distance ofabout 100 nm from the slit 207.

Next, in the step shown in FIG. 7(e), a metal such as aluminum is buriedin the grooves 208 and 209 to form the collector contact 201 b and theemitter contact 203 b. After a polysilicon film heavily doped with boronis deposited on the substrate, it is patterned to form the external baselayer 202 b in contact with the internal base layer 202 a within theslit 207.

In the lateral heterojunction bipolar transistor according to thepresent embodiment, the internal base layer 202 a is composed of theSiGe layer formed by epitaxial growth so that a heterojunction having arelatively sharp concentration profile is formed as described above.

In addition, the internal base layer 202 a and the external base layer202 b are connected to each other by self alignment in accordance withthe method of the present embodiment. This reduces a parasiticresistance and reduces a parasitic capacitance particularly remarkably.Since the collector contact 201 b and the emitter contact 203 b can becomposed of the buried metal, the parasitic resistance of each of thecontacts is reduced, which allows the formation of a lateralheterojunction bipolar transistor with excellent characteristics.

Embodiment 4

FIG. 8 is a plan view of a lateral heterojunction bipolar transistoraccording to a fourth embodiment of the present invention. The lateralheterojunction bipolar transistor of the present embodiment hasbasically the same plan configuration as that of the third embodiment,though the plan view thereof is not shown in the present embodiment.

As shown in FIG. 8, the lateral heterojunction bipolar transistor of thepresent embodiment has basically the same structure as that of the thirdembodiment except that the emitter 203 a and the collector 201 a havedifferent impurity concentrations.

In the present embodiment, the emitter 203 a is doped with antimony (Sb)at a high concentration of about 1×10²⁰ atoms·cm⁻³ or more and thecollector 201 a is doped with antimony (Sb) at a relatively lowconcentration of about 1×10¹⁷ atoms·cm⁻³. By thus doping the emitter 203a and the collector 201 a with the impurity at the respective optimumconcentrations, electrons can be implanted efficiently from the emitter203 a into the collector 201 a through the internal base layer 202 a,which achieves the effect of implementing a high-speed and high-gaintransistor operation in addition to the effects achieved by the thirdembodiment.

Referring to FIGS. 9(a) to 9(f), a method of fabricating the lateralheterojunction bipolar transistor according to the present embodimentwill be described. FIGS. 9(a) to 9(f) are cross-sectional viewsillustrating the process of fabricating the lateral heterojunctionbipolar transistor according to the present embodiment.

First, in the step shown in FIG. 9(a), the SOI substrate composed of theSi substrate 250, the BOX layer 251 composed of the silicon oxide film,and the Si film (semiconductor layer) formed on the BOX layer 251 isformed. The semiconductor layer 252 has a thickness of about 200 nm.Then, a resist mask 220 having an opening wider than a combined regionof an emitter formation region and a collector formation region isformed on the semiconductor layer 252. Subsequently, antimony (Sb) ionsare implanted into the semiconductor layer 252 from above the resistmask 220 under such conditions that the concentration of antimony in thesemiconductor layer 252 is about 1×10¹⁷ atoms·cm⁻³. By the step, alow-concentration impurity implanted region 210 which is to serve as acollector later and a high-concentration impurity implanted region 211which is to serve as an emitter are formed in the semiconductor layer252.

Next, in the step shown in FIG. 9(b), the oxide film 206 is formed onthe semiconductor layer 252. Then, a resist mask 221 having an openingincluding the emitter formation region of the region into which antimonyions have been implanted and overlapping a region in which a slit forbase formation is to be formed is formed on the oxide film 206.Subsequently, antimony ions (Sb) are implanted into the semiconductorlayer 252 from above the resist mask 221 under such conditions that theconcentration of antimony in the semiconductor layer 252 is 1×10²⁰atoms·cm⁻³.

Then, in the step shown in FIG. 9(c), a slit 207 is formed in the middleportion of the oxide film 206. The slit is then increased in depth topenetrate the semiconductor layer 252 in the step shown in FIG. 9(d).

Next, in the step shown in FIG. 9(e), Si_(0.7)Ge_(0.3) is epitaxiallygrown by CVD or UHV-CVD from both sides of the slit 207 to be united inthe middle of the slit 207, thereby forming the internal base layer 202a buried in the slit 207. During the step, the internal base layer 202 ais doped in situ with boron at a concentration of about 5×10¹⁸atoms·cm⁻³. Thereafter, grooves are formed by dry etching in the regionsof the oxide film 206 located on both sides of the slit and at adistance of about 200 nm therefrom. The grooves are further enlarged bywet etching such that one of the grooves reaches the semiconductor layer252 and the low-concentration impurity implanted region 210 to form agroove 208 therein and the other of the grooves reaches thesemiconductor layer 252 and the high-concentration impurity implantedregion grooves 211 to form a groove 209 therein. During the step, theisotropic etching action of wet etching laterally enlarges the grooves208 and 209 till the end portion of each of the grooves 208 and 209reaches a point at a distance of about 100 nm from the slit 207.

Next, in the step shown in FIG. 9(f), a metal such as aluminum is buriedin the grooves 208 and 209 to form the collector contact 201 b and theemitter contact 203 b. After a polysilicon film heavily doped with boronis deposited on the substrate, it is patterned to form the external baselayer 202 b in contact with the internal base layer 202 a within theslit 207.

In the lateral heterojunction bipolar transistor according to thepresent embodiment, the impurity concentrations in the emitter 203 a andthe collector 201 a can be adjusted to be more suitable for theoperation of the bipolar transistor. This achieves the effect ofproviding, by simple process steps, an impurity concentration profile inwhich the emitter 203 a and the collector 201 a have differentconcentrations in addition to the effects achieved by the thirdembodiment.

What is claimed is:
 1. A lateral heterojunction bipolar transistor,comprising: a substrate having an insulating layer; a firstsemiconductor layer in a mesa configuration disposed on the insulatinglayer; a second semiconductor layer formed by epitaxial growth on a sidesurface of the first semiconductor layer, the second semiconductor layerhaving a band gap different from a band gap of the first semiconductorlayer; and a third semiconductor layer formed by epitaxial growth on aside surface of the second semiconductor layer, the third semiconductorlayer having a band gap different from the band gap of the secondsemiconductor layer, at least a part of the second semiconductor layerfunctioning as an internal base layer of a second conductivity type,wherein a principal surface of the first semiconductor layer is a {110}plane and a side surface of the first semiconductor layer in contactwith the second semiconductor layer is a {111} plane.
 2. A lateralheterojunction bipolar transistor disposed on an insulating layer, thetransistor comprising: a first semiconductor layer functioning as acollector; a second semiconductor layer disposed in contact with atleast one side surface of the first semiconductor layer to function asan internal base having a band gap smaller than a band gap of the firstsemiconductor layer; a third semiconductor layer disposed in contactwith a side surface of the second semiconductor layer to function as anemitter having a band gap larger than the band gap of the secondsemiconductor layer; first and second electrodes in contact withrespective side surfaces of the first and third semiconductor layers;and a third electrode disposed in contact with a top surface of thesecond semiconductor layer.
 3. The lateral heterojunction bipolartransistor of claim 2, wherein each of the first and second electrodesis composed of a metal.